Lighting control in an office or commercial building has gone through several stages ranging from “on/off”-control of a single lamp or a group of lamps, through dimming of a single lamp or a group of lamps, to advanced control of the lighting in an entire building. In normal lighting systems it is the individual lamp drivers of the luminaires that are controlled. The traditional lamp driver systems are wired control interface systems, such as the standard 1-10 V dimming interface and more recently digital systems, such as the Digital Addressable Lighting Interface (DALI). The interface systems are moving towards wireless interfaces, such as systems implementing the ZigBee standard, a system that uses radio frequencies around 2.4 GHz.
A wireless interface using electromagnetic signals needs an antenna for transmission and reception of control signals. For a lamp driver the situation is, however, complicated by the fact that the application has a metal housing. The housing will isolate an internal antenna from the environment, thereby largely blocking the transmission and reception of the electromagnetic signals. Furthermore the lamp driver itself can be enclosed in a metal housing that further attenuates the electromagnetic signals. In some technological areas this is not a problem. For example, a cellular phone has a plastic housing and the antenna can be located completely inside the application.
In the US patent application 2003/0090889 a ballast with an integrated RF wireless interface is disclosed. For a ballast with an embedded antenna, it is disclosed that in order to get the radiation outside the ballast, a plastic case may be used as a cover for the ballast, or in case of a metal cased ballast, a halfwavelength slot antenna may be used as the cover.